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Saturday, 1 April 2017

Halftrack Experiments

Ever since their appearance in the mid-1910s, halftracks have been considered as a chassis for armoured vehicles, especially SPGs. Better off-road performance than wheeled vehicles and stability made these vehicles an attractive chassis for artillery. Halftrack SPGs were popular in Germany and the United States. The heroes of this article, Soviet ZIS-41 and ZIS-43 halftracks, are not as well known.

Bigger Chassis

Wheeled SPGs (29-K SPAAG on the YaG-10 chassis and light SU-1-12 on the GAZ-AAA chassis) were the only kind of SPG that was built en masse in the USSR in the 1930s. As for halftracks, artillery designers were not interested. The reason was simple: the USSR had no halftracks that could be used as an SPG chassis.

The Scientific Automotive Tractor Institute (NATI) managed to produce halfway decent halftracks only towards the end of the 1930s. The GAZ-AA based halftrack received the index GAZ-60, and the ZIS-5 halftrack was called ZIS-22. Both vehicles were in mass production by 1941, but the military showered them in complaints. An increase in off-road performance came at the cost of higher fuel consumption, and the speed of halftracks was much lower than that of wheeled trucks.

The problems with the halftracks were obvious, and it's not surprising that NATI, as well as GAZ and ZIS factory design bureaus, actively worked on all wheel drive trucks. ZIS was working on ZIS-32 and ZIS-36 trucks, which were supposed to replace the ZIS-5 and ZIS-6. GAZ worked on GAZ-63 and GAZ-33 trucks, which would replace the GAZ-AA and GAZ-AAA.

Trials of the 37 mm 61-K AA autocannon on the chassis of a ZIS-32 truck, July of 1941

GABTU could see that these vehicles were much better than halftracks. It's not surprising that in May of 1941, when work on SPGs was discussed, the ZIS-22 halftrack was absent from the list of prospective chassis. However, the idea of installing the 25 mm 72-K autocannon in the ZIS-32 or ZIS-36 truck bed was raised.

Work in this direction went further than just theory. ZIS-32 and ZIS-36 trucks equipped with the 37 mm 61-K AA autocannon began trials in early July. Trials went successfully. However, the ZIS-36 never entered mass production, and the ZIS-32 did not last long on the assembly line, and only as a regular truck. Nevertheless, this was a signal to look into development of analogous SPAAGs on the mass produced GAZ-AA, GAZ-AAA, and ZIS-5 truck chassis.

Halftracks were also absent from the list of factory #92 projects. By July 20th, 1941, the design bureau prepared prototypes of 57 mm anti-tank gun armed SPGs. The ZIS-30 was built on the chassis of the Komsolomets tractor, and the ZIS-31 was built on the GAZ-AAA chassis with an armoured cabin. The ZIS-30 turned out better, since the Komsomolets performed better off-road. That is when Grabin turned his sights to a halftracked chassis.

ZIS-22M halftrack, July of 1941. Grabin picked this vehicle as the chassis for his tank destroyer.

The GAZ-60 could not be used as a chassis, as it could barely pull itself along. The ZIS-22 was a different story. An improved version of this vehicle, indexed ZIS-22M, entered trials in June of 1944. It had the improved ZIS-16 engine and an improved radiator from the ZIS-101. The suspension was also modernized. The vehicle performed better than the ZIS-22 on trials, and it was recommended for mass production. This was the vehicle that factory #92 decided to use as the chassis for its new SPG. The project was not Grabin or Muravyev's personal initiative. The work was overseen by the Directorate of Ground Armament of the Main Artillery Directorate (UVNA GAU KA).

ZIS-41 on trials, April of 1942

Tactical-technical requirements for a "57 mm anti-tank SPG on the chassis of a ZIS-22M all-terrain vehicle" were finalized on August 21st, 1941. The accompanying letter, copies of which were sent to ZIS and factory #92, indicated that the changes to the ZIS-22M chassis should be minimal. In addition, the chassis had to be returned to ZIS after the design was complete. That was done because the ZIS-22M existed in very small numbers at the time.

According to the requirements, the chassis had to have armour for the cabin and engine compartment, as well as the fuel tank and ammunition racks. The mass had to be no more than 6.5 tons, and the range at least 250 km. Aside from a ZIS-2 gun, the halftrack had to have a DT machinegun in the cabin. The SPG had to carry at least 100 shells, which had to be easy to load. The rate of fire was estimated at 15 RPM.

Crossing a snowy slope.

The appearance of the SPG that was indexed ZIS-41 was no accident. Only 100 ZIS-30s were produced, and the Komsomolets tractor was out of production by August of 1941, so no more could be built. There was no alternative to the ZIS-22M. The problem was that production of the ZIS-22M was also not going smoothly. Only three were built, and an attempt to mass produce it in Moscow failed.

The index ZIS-41 was given to the SPG by factory #92, not the automotive factory in Moscow.

The design of the cooling grilles can be seen in this photo. Trials showed that the design was poor.

It's worth mentioning a few details about the work done in 1941. A few authors write that the vehicle was built and tested in the fall of 1941, and then work stopped before the end of the year. This is not the case. The SPG was definitely built on the ZIS-22M chassis. The question is when it was built. The halftrack itself received the name ZIS-42, and one of the experimental vehicles was equipped with an M-13 rocket launcher system. This vehicle went through testing in mid-September alongside analogous launchers on the T-40 and STZ-5 chassis.

ZIS began its evacuation from Moscow in October of 1941. The question of whether or not the ZIS-41 was finished before then is still open. However, it is obvious that the information about ZIS-41 trials in November is nonsense. The real trials happened several months later.

The gun platform design can be seen in this photo.

According the the experimental work schedule dated February 1st, 1942, the experimental SPG was ready by then, but trials have not yet begun. The Artillery Committee was still preparing the trials program. 15,239 rubles were spent on designing the ZIS-41, 28,255 were spent on building the prototype, and the ZIS-2 gun cost 11,500 rubles.

In addition, it was planned to build 24 ZIS-41s in the spring of 1942, before the trials were even complete. The due date was October of 1942. Factory #92 and ZIS were responsible for production. The budget was 2,400,000 rubles, or 100,000 per SPG. This was not cheap: the light T-60 and T-70 tanks, as well as SPGs on their chassis, cost less.

Nevertheless, the documents prove that work on the ZIS-41 did not end in 1941 and trials were not held in 1941.

With many inconveniences

The ZIS-41 was tested at the Gorohovoets artillery proving grounds (ANIOP). The orders for trials and the trials schedule arrived on February 24th, 1942. The SPG itself came later, on March 13th, and the documentation came a week later. One of the experimental ZIS-22M prototypes was used to build the SPG. It was rather experienced: according to documents, it had already traveled 16,500 km when the mobility trials began.

Factory #92 did not meet the weight requirements. The mass of the ZIS-41 was 7.5 tons instead of 6.5 tons. To be fair, it was not possible to meet them. Conversion of the GAZ-AAA into the ZIS-31 already raised the mass from 2.5 to 5 tons. Since the ZIS-22M had a mass of 5150 kg, it was not possible for the SPG to weigh less than 7.5 tons.

The driver in the ZIS-41 sat like this. No comment.

As required, the SPG received an armoured cabin that covered both the engine and driver's compartment. The cabin held the driver and the machinegunner, although the experimental vehicle had no DT mount. The front armour was sloped. The driver's compartment was not that well thought out. A special report regarding the crew positions was dedicated to this.

Traditionally for Grabin's designs, the crew's comfort came last. There was no space left for the driver's legs as a result. The driver sat bunched up with his knees up to the steering wheel. It would be interesting to see how long someone could drive like this.

Military Doctor Second Class Aleksandrov, who studied the crew workspaces on the ZIS-41, clearly did not think that the driver had to endure all difficulties and adversities of military life, and simply stated that the driver's cabin had to be redone to make room for his legs. However, factory #92 never made this change.

ZIS-41 with the gun removed.

The fighting compartment was significantly different from the one on the ZIS-30. Factory #92's designers took the truck bed, rejecting the flip-open sides. Instead, they added small guard rails around the edges, which served a rather symbolic purpose. The platform itself was made from armoured steel and had a layer of wooden planks above it. The ZIS-2 gun was mounted on a foundation in the middle of the platform, giving it 360 degree traverse. A gun shield was added to protect the crew. If flipped up, the crew could work while standing.

The requirement to carry 100 shells was met in a very unusual way. The SPG had no ammunition racks. The shell crates were laid directly on the platform. Aside from a loader and a gunner, the gun crew included an observer, who also stood on the platform.

Foundation of the gun.

Various modifications led to trials being postponed until April 14th. These were just the mobility trials, gunnery had to wait. The ZIS-41 could remove its gun and be turned into an artillery tractor, and tractors were a pressing issue.

The GAU was working on a half-armoured tractor for divisional artillery at the time. The ZIS-41 was potentially suitable for this role. Plus, the ZIS-42 was built in late April of 1942, which was tested by towing a 122 mm M-30 howitzer. The vehicle succeeded, pulling the gun behind it off-road.

Overall view of the gun shield.

The ZIS-41 entered a new round of trials on April 24th, approved by the Deputy People's Commissar of Defense, Colonel-General N.N. Voronov and the GAU Chief, Colonel-General N.D. Yakovlev. The gun and foundation were removed. Instead, 30 crates of shells for the M-30 and 8 crewmen were put into the truck bed. The howitzer was attached to the rear. In addition to 3 tons in the truck bed, the halftrack pulled another 2.5 tons behind it. The halftrack drove for 192 km between April 24th and April 27th, 8 of them across the snow with skis on its wheels, 145 on a highway, and 49 off-road.

The tractor accelerated up to 37 kph, spending 50 L of fuel for 100 km. On dirt roads, the speed fell to 12-30 kph, in second or third gear. Off-road, the top speed was 5-8 kph, and the fuel expenditure rose to 200 L per 100 km.

ZIS-41 as an artillery tractor.

The effectiveness of the front wheels dropped noticeably off-road. To solve this problem the designers installed tractor type brakes. A similar solution was used on German halftracks. The turning radius was also high: 10-15 meters on the highway, 30-40 off-road. The "regular" ZIS-42 had the same issues.

Another issue was overheating of the engine, which was caused by a poor design of the grille armour and overloading of the vehicle. This was particularly noticeable off-road, when the water in the cooling system began boiling every 2-3 km. The situation on a road was not much better: here the water boiled every 10-15 km. For comparison, the ZIS-42 did not overheat in these conditions. The testers also did not like the lack of normal truck bed walls. The vehicle was deemed conditionally suitable for use as a tractor, but with many cautions.

Mobility trials. It's unlikely that the crew took too much pleasure in riding on the ammunition crates.

Firing trials could only start on May 27th, 1942, and lasted until June 6th. 371 shots were fired, 163 with increased propellant load. The gun itself worked fine, but many complaints were made about the SPG. The lack of an ammunition rack reduced the mobility of fire. The loader had to leave the cover of the gun shield to get ammunition, which put him at risk of being hit by bullets or shrapnel. Another issue was that the gases from the gun barrel could smash the headlights or enter the cabin.

The high bore axis meant that the SPG would start to oscillate when firing. This was specifically noticeable when firing at 90 degree angles. The gunner suffered from oscillations. The report said that his seat was uncomfortable and that he had to hold onto the flywheels in order to keep himself from falling off.

The conclusions were as follows:

"Based on the results of the trials, the Gorohovets ANIOP considers that the 57 mm ZIS-41 SPG requires additional design improvements.

The Gorohovets ANIOP considers the main drawbacks of the SPG the poor stability after firing at a 90 degree angle (the platform tilts at a high angle), unsatisfactory engine cooling, and a lack of ammunition racks. The factory needs to correct these drawbacks, repeat the side shooting trials, and perform additional mobility trials."

Despite the negative verdict, the story of the ZIS-41 was not finished. The GAU liked the idea of installing an anti-tank gun on the ZIS-42, but the vehicle needed a lot of work. In addition, the ZIS-2 was also in trouble. The gun was removed from production in late 1941, and the 76 mm divisional gun mod. 1942, also known as the ZIS-3, took its place. Coincidentally, that gun was designed to use the ZIS-2 mount.

Many battles were fought over the reason for the cancellation of the ZIS-2. Some blame excessive penetration, others have their own theories. The real reason was complicated. First of all, production of barrels, and more importantly, ammunition, for this gun was complicated. In addition, factory #92 was loaded with other orders.

With the ZIS-3 that was initially not accepted into production, the situation was different. The ballistics were the same as the already popular USV and F-34 guns, so there were no unusual problems with its production.

In addition, the ZIS-2 was the target of complaints from antitank gunners. They complained that the gun was too heavy and its trails were too long. The GAU made the decision to develop a similar gun, but with a shorter barrel and a muzzle velocity of 900 m/s. The gun, with a 63 caliber barrel and a ZIS-3 mount, was presented by factory #92 in July of 1942. It was indexed IS-1 and passed factory trials successfully. This gun became the priority for the ZIS-41. However, the Gorohovets proving grounds found a number of defects of the IS-1, and was not put into mass production.

As a result, the GAU was forced to resurrect the ZIS-2 in the spring of 1943. In the summer of 1943, the gun was back in mass production.

Firing trials. The sides of the engine compartment are open, but this did not help against overheating.

The issues weren't only with the gun. ZIS was evacuated in October of 1941 and was busy with restoring the production of trucks. The ZIS-5 was a priority, and nobody had time for the ZIS-42. The prospective SPG remained without a chassis.

The factory didn't exactly rejoice at the thought of working on the ZIS-41, and there were good reasons for it. As a result, the work on the ZIS-41 was moved to factory #592 in mid-October of 1942. Its designers already had experience with wheeled SPGs, but their work did not move past paper. This proposal was initiated by the Artkom, who approved them with letter #543460 from September 22nd.

According to the requirements, the SPG had to use either the 57 mm IS-1 or 76 mm ZIS-3 gun. The chassis was still the ZIS-42. However, new orders were given on October 31st. ZIS management agreed to finish working on the ZIS-41, and the project was taken away from factory #592. However, work on the ZIS-41 did not continue in Moscow and the project eventually vanished.

No need for armour

The waning of work on the ZIS-41 coincided with the start of work on another SPG on the ZIS-42 chassis. The first information about it crops up in early November of 1942. The Moscow ZIS factory interpreted the word "improvement" in a very creative way. In reality, the factory started working on a whole new vehicle that used the experience from the ZIS-41. This time, the vehicle was not a tank destroyer, but a SPAAG. It received the index ZIS-43. This is an index given by the automotive factory, not factory #92's design bureau. History made a full circle. The halftrack SPG returned to where it started from: ZIS.

ZIS-43 SPG in travel position, December of 1942.

On November 14th. 1942, by request of ZIS director Likhachev, the Artkom sent tactical-technical requirements for a "37 mm AA gun on the ZIS-42 all-terrain vehicle". According to them, the cabin and engine were covered in armour, and the AA gun received a shield. The requirements specifically said that the cabin had to be designed in such a way that the driver was comfortable. The need to change the airflow in the engine compartment was also stated. The GAU once again insisted that the weight of the SPG had to be less than 6.5 tons. It's difficult to say how the artillerymen expected to lower the mass with the same armour and the requirement to have a gun platform with walls.

The same SPG in travel position.

ZIS did not let development drag on, and the ZIS-43 made it to trials by December 15th, 1942. This time, the new ZIS-42 chassis was used. Armour steel was not wasted on the prototype, and boilerplate sufficed. The engine and cabin armour looked the same, but with some nuances. The design of the grilles changed, and the airflow was better. The mass of the SPG grew even more, to 8750 kg, so the load on the engine increased.

Hatches in the sides of the engine compartment were added to make maintenance easier, as well as a large two-piece hatch on top. The cabin was redesigned, as there were no longer any complaints about the driver's conditions. This time, there was no coaxial machinegun. The truck bed, ammunition racks, and gun mount were taken from the ZIS-5 SPAAG. The only difference was that the 37 mm 61-K autocannon now had a gun shield.

The new grille design is visible.

The ZIS-43 drove to the proving grounds on its own, traveling 380 km from the factory to the Gorohovets proving grounds. The SPG drove for another 500 km as a part of the trials from December 19th to the 23rd. The engine overheated twice, but there was a new problem. The temperature in the cabin reached 40-50 degrees while driving. The floor in front of the commander's seat was hot. If this happened in December, when the air temperature was -10 °C, it's hard to imagine what would have happened during the summer.

The increased mass took its toll on the top speed and fuel efficiency. The SPG could not go faster than 35 kph, and the average speed was 29.3 kph. The fuel expenditure grew to 90 L per 100 km. The average speed on dirt roads was 19.5 kph, and the fuel expenditure was 110 L per 100 km.

Skis installed during movement across snow are visible on the rear wall of the truck bed.

The results of the firing trials were the same as seen on the ZIS-41 earlier. The headlight was smashed by gases. The high center of gravity affected the firing precision, especially when firing to the sides. Nevertheless, the stability of the platform was deemed sufficient. In addition, the testers rejected the travel lock design.

The following conclusion was given as a result of the trials.

"The 37 mm AA autocannon installed on the modernized ZIS-42 chassis can be used for AA defense of motomechanized unit on the march in areas where enemy anti-tank forces are not present. Due to its mobility and off-road performance, its main objective is to cover all types of forces on the march.

The large size, thin armour, and poorer mobility than the stock vehicle does not allow it to replace the 37 mm SPAAG on a tracked chassis (factory #38 type).

The experimental prototype of the 37 mm AA autocannon passed trials.

After all defects mentioned in the report are corrected, both in the gun and chassis, the commission can recommend the prototype for service in the Red Army."

The fate of the ZIS-43 was decided quickly. After studying the results of the trials, the GABTU armour directorate (BTU) proposed that further work be cancelled. Since the SPAAG was meant to protect units on the march, there was no sense in having armour. This also solved the issue of overloading the chassis. There was no need for the ZIS-43.

However, the BTU chief, Engineer-Colonel Afonin, raised the question of a vehicle analogous to the ZIS-41. This time, matters did not proceed past proposals. As for the ZIS-43, it had its armour removed in the spring of 1943, and it turned into an ordinary ZIS-42, without notifying the GAU.

The Red Army did receive a halftracked SPG with a 57 mm gun, and later even a SPAAG halftrack. Of course, they were called GCM T48 and MGMC M17, and Soviet design bureaus had nothing to do with them. These vehicles began arriving through the Lend Lease program starting in late 1943.